1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory and a method of manufacturing the nonvolatile semiconductor memory.
2. Related Art
There is a nonvolatile memory of a double gate structure in which there are provided a floating gate electrode and a control gate electrode positioned above the floating gate electrode, a magnetic field is applied by this control gate electrode and a charge is injected into the floating gate electrode or a charge is extracted from the floating gate electrode by Fowler-Nordheim tunneling, thereby permitting electrical writing or erase of data.
In this structure, a polycrystalline silicon layer that becomes the floating gate electrode is sandwiched between a tunnel insulating film and an interpoly insulating film, and a charge is injected into the floating gate electrode by F-N tunneling of electrons from a silicon substrate. As a result of this, the inversion threshold voltage of a channel region is adjusted in association with the charging of the floating gate electrode and the on-off condition of a transistor is determined.
In such a nonvolatile memory, the reliability of an insulating film is important. In this reliability, the effect of a bird's beak becomes great in association with the miniaturization of elements. In the post-oxidation step after the formation of a tunnel insulating film and a floating gate electrode, an oxidizer enters the interior from a gate end portion of a tunnel insulating film and oxidizes the surface of a silicon substrate where a channel is formed and the surface of a floating gate electrode formed by polycrystalline silicon, whereby this bird's beak is formed.
When a bird's beak has been formed, the film thickness of that place becomes large and a tunnel current does not flow easily any more. As a result of this, a tunnel current concentrates in a flat area of a thin film where no bird's beak has been formed, the film quality of that place deteriorates, and insulating film reliability decreases.
When the channel length and channel width of a transistor are large, the concentration of a tunnel current has a small effect because the flat area of a thin film covers a wide region of the insulating film even when a bird's beak has been formed. However, the effect of a bird's beak increases as miniaturization becomes to a greater extent.
Furthermore, it is difficult to control the amount of a bird's beak. A bird's beak causes variations in the capacitance of a tunnel insulating film, and these variations contribute to the deterioration in the performance of a device.
Because the surface side of a floating gate electrode that is formed by polycrystalline silicon having a high impurity concentration is oxidized thicker than the silicon substrate surface where a channel is formed, it is necessary to suppress the oxidization of the floating gate electrode in order to suppress the formation of a bird's beak.
There has been proposed a method of suppressing the formation of a bird's beak in an interpoly insulating film, though not a tunnel insulating film (refer to Japanese Patent Laid-Open No. 7-249697, for example). This method involves controlling the formation of a bird's beak by using an NONON-film formed by inserting a silicon nitride film each above and below an ONO-film (a stacked film composed of an oxide film-nitride film-oxide film) as an interpoly insulating film.
However, because the side surfaces of the floating gate electrode and the side surfaces of the control gate electrode are not nitrided, in the post-oxidation step after the processing of the transistor gates, the side surfaces of the floating gate electrode and the control gate electrode are oxidized, resulting in diminished sizes.
Because the polycrystalline silicon that forms the gate electrode has various planar direction dependences, the oxidation amount in post-oxidation varies from cell to cell. That is, variations in gate size occur for each cell.
Therefore, variations in the oxidation amount of the side walls of the polycrystalline silicon that forms the floating gate electrode cannot be suppressed simply by inserting silicon nitride films into the interface of the interpoly insulating film even if the formation of a bird's beak can be suppressed thereby.
The variations in cell shape lead to variations in the characteristics of a cell transistor. In order to absorb variations in the characteristics of a transistor, it becomes necessary to sacrifice other performance capabilities of a semiconductor device, for example, it becomes necessary to lower the writing speed. All things considered, this leads to the deterioration of the performance of a semiconductor device. That is, a reduction of variations in cell shape is a very important item of technology development.